The number of individuals with unilateral transtibial amputation (TTA) is rapidly growing in the United States, and these individuals suffer from a variety of gait deficits, including a greater risk and fear of falling. Falls are a serious health problem, and account for approximately 30% of injury-related medical costs in the U.S. Walking on sloped surfaces and on stairs is frequently encountered in daily living activities and presents a greater risk of falling relative to walking on level ground, particularly for people with disabilties, such as individuals with amputation. Recently, whole-body angular momentum has been investigated during level, unimpaired walking, and must be regulated in order to maintain dynamic balance. Analysis of additional biomechanical measures, such as the external moment and margin of stability, can result in a comprehensive view for dynamic balance when combined with whole-body angular momentum. In addition, powered prostheses have recently become commercially available, and have shown promising results in reducing the metabolic cost of walking. However, the effects of functionally different prostheses on dynamic balance during walking on varied surfaces remain unclear. The proposed work aims to quantify dynamic balance in TTA using both passive and powered prostheses relative to non-amputees during walking on stairs and on sloped surfaces. Specific Aim 1: Quantify differences in whole-body angular momentum, external moment and margin of stability between TTA using a passive prosthesis, using a powered prosthesis and non-amputees during stair ascent and descent to determine the effects of functionally different prostheses on fall risk Specific Aim 2: Quantify differences in whole-body angular momentum, external moment and margin of stability between TTA using a passive prosthesis, using a powered prosthesis and non-amputees during incline and decline walking on slopes of zero, five and ten degrees to determine the effects of functionally different prostheses on fall risk. Through these specific aims, the effects of a lower limb amputation and functional changes in the prosthesis on dynamic balance will be determined. The angular momentum, external moment and margin of stability results will be further interpreted using joint kinetics to identify biomechanical mechanisms that may contribute to balance control. The proposed work provides a foundation to substantially extend this research to experimental and computational studies of balance control, muscle and prosthesis function, and altered prosthesis control strategies. In addition, the approach can be extended to additional patient populations. The long-term objectives of this work are to improve mobility, productivity and independence in individuals with lower-limb amputation by reducing fall risk.